1. Field of the Invention
The present invention relates to a field effect transistor, and particularly to a device having a quantum well in a channel structure. The channel formed along the quantum well is two-dimensional due to confinement of carriers in the quantum well.
2. Description of the Related Art
Many attempts have been made to improve the characteristics of a field effect transistor FET in realizing a high transconductance (gm), in reducing a short channel effect and so on. The basic GaAs MESFET has a semi-insulator GaAs substrate, an n-GaAs layer, n.sup.+ layers contacting with a source and a drain respectively, source and drain electrodes, and a gate electrode. When a bias voltage is applied to the gate electrode, a depletion layer is extended to control a current flow. At this time, if a channel length is shortened, there will be caused a problem on the short channel effect. Generally when the channel length is about 1 .mu.m or less, a threshold voltage V.sub.th of the field effect transistor is positively changed. In this case the larger the impurity concentration N in an active layer of the channel, the smaller the change of the threshold V.sub.th. Therefore, in order to reduce the short channel effect in a MESFET, the active layer may be highly doped. Once the active layer is highly doped, the depletion layer is made thinner so that the gm may be improved.
In the prior art device, however, the number of carriers cannot be simply increased by employing high doping due to such problems as a decrease in electron mobility and a decrease in a device withstand voltage.